1. Field of the Invention
This invention relates in general to earth drilling equipment and in particular to rotary power slips mounted concentrically with the pipe opening in a rotary drilling table. It is particularly useful for running drill pipe in and out of a well bore and for running casing pipe into a well bore.
2. Description of the Prior Art
In the past, rotary power slip assemblies utilized slips operated by a fluid cylinder mounted in a stationary position on the rotary table. This arrangement necessitated the use of a mechanical linkage between the fluid cylinder and rotary housing which was subject to fouling and malfunctioning. The slip assembly could not be activated at any point in its rotation but required alignment of the fluid cylinder and rotary housing. The assembly protruded above the rig floor thus consuming valuable space.
In U.S. Pat. No. 3,999,260 to Terry E. Stuckey et al, entitled "Rotary Power Slip Assembly", issued Dec. 28, 1976, rotary power slips are disclosed in which the slips are operated by means of a fluid cylinder carried by the rotary housing. Fluid pressure is first supplied to a stationary seal ring. Expansive seal means on the stationary seal ring form a fluid duct when pressure is applied. Pressure then flows through the inflated duct to the fluid cylinder on the rotary housing to operate the slips. Stuckey's power slips mounted flush with the drill rig floor, eliminated the linkage problems, and could be operated at any point in their rotation. In spite of these advantages, Stuckey's power slips required a cumbersome mounting structure underneath the rotary table to support the stationary seal ring. The expansive seal means deteriorated rapidly due to rig vibration affecting alignment of the seal means and rotary housing. Mud and debris also collected on the surface of the expansive seal.